Metal material bonding method

ABSTRACT

In a metal material bonding method, an insertion material, which has a lower melting point than that of metal materials to be bonded, is interposed between bonding end surfaces of the metal materials to be bonded. The metal materials to be bonded is heated and held to a temperature not lower than the melting point of the insertion material and not higher than the melting point of the metal materials to be bonded while applying pressure to bonding surfaces of the metal materials to be bonded. The insertion material is formed to have a size which is smaller than each of the bonding surfaces of the metal materials to be bonded.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a metal material bonding method. More specifically, it relates to improvement in a metal material bonding method in which an insertion material is interposed between the metal materials to be bonded.

2. Description of the Related Art

Heretofore, a bonding method using an insertion material interposed between materials to be bonded to each other has been employed in liquid-phase diffusion bonding.

However, because the insertion material used is formed to have a shape and a size so as to cover the whole of bonding surfaces, there arises a problem that the insertion material is partially extruded between materials to be bonded at the time of bonding so that surfaces of the bonding portions of the materials to be bonded are covered with the insertion material. When the bonding portions are under such a condition, the insertion material extruded and solidified so as to cover the surfaces of the bonding portions of the materials to be bonded exerts a notching effect so that strength against fatigue of bonding is lowered. Therefore, the work of removing the insertion material covering the surfaces of the bonding portions of the materials to be bonded after bonding is required. There arises also a problem that this brings about lowering of productivity or workability.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a metal material bonding method in which an insertion material interposed between bonding surfaces of the materials to be bonded is prevented from excessively covering bonding end portions of the materials to be bonded at the time of bonding.

According to the present invention, the metal material bonding method comprises the steps of: interposing an insertion material, which has a lower melting point than that of metal materials to be bonded, between bonding end surfaces of the metal materials to be bonded; and heating and holding the metal materials to be bonded to a temperature not lower than the melting point of the insertion material and not higher than the melting point of the metal materials to be bonded while applying pressure to bonding surfaces of the metal materials to be bonded, wherein the insertion material is formed to have a size which is smaller than each of the bonding surfaces of the metal materials to be bonded.

According to a first embodiment of the metal material bonding method of the present invention, each of the metal materials to be bonded is provided as a solid material; the insertion material is formed of an Ni-group or Fe-group alloy containing boron in a range of from 2.0 mass % to 5.0 mass % (preferably, 3.0 mass % to 4.0 mass %) and has a thickness in a range of from 20 μm to 100 μm (preferably, 30 μm to 50 μm); the ratio of the area of the insertion material to the area of each of the bonding surfaces of the metal materials to be bonded is in a range of from 50% to 99% (preferably, 70% to 85%); and the distance between the outer edge of the insertion material and the outer edge of each of the metal materials to be bonded is not smaller than a value ten times (preferably, fifteen times) as large as the thickness of the insertion material.

According to a second embodiment of the metal material bonding method of the present invention, each of the metal materials to be bonded is provided as a hollow material; the insertion material is formed of an Ni-group or Fe-group alloy containing boron in a range of from 2.0 mass % to 5.0 mass % (preferably, 3.0 mass % to 4.0 mass %) and has a thickness in a range of from 20 μm to 100 μm (preferably, 30 μm to 50 μm); the ratio of the area of the insertion material to the area of each of the bonding surfaces of the metal materials to be bonded is in a range of from 50% to 99% (70% to 85%); the distance between the outer edge of the insertion material and the outer edge of each of the metal materials to be bonded is not smaller than a value ten times as large as the thickness of the insertion material; and the distance between the inner edge of the insertion material and the inner edge of each of the metal materials to be bonded is not larger than a value one hundred times as large as the thickness of the insertion material (preferably, the inner edge of each of the metal materials to be bonded is not smaller than a value ten times as large as the thickness of the insertion material).

In the metal material bonding method of the present invention, it is preferable that the surface roughness R_(max) of each of the bonding surfaces of the metal materials to be bonded is not larger than 50 μm; the pressure applied to the bonding surfaces of the metal materials to be bonded is in a range of from 3 MPa to 9 MPa; and the heating and holding is performed in a non-oxidative atmosphere.

In the metal material bonding method of the present invention, it is preferable that the heating is performed by means of induction heating or high frequency resitance heating.

In this case, it is preferable that the frequency of a current in the induction heating or resistance heating is in a range of from 3 kHz to 100 kHz (preferably, 3 kHz to 30 kHz).

In the metal material bonding method according to the present invention, the size of the insertion material is adjusted so that the insertion material is set inward by a predetermined quantity from the outer edge of each of the bonding surfaces of the metal materials to which the insertion material is bonded. Accordingly, a part of the insertion material is prevented from being excessively extruded to the bonding end portions at the time of bonding. Accordingly, the strength against the fatigue of the bonding portions is prevented from being lowered by the insertion material which is extruded to the surfaces of the bonding portions and solidified thereat.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a schematic view of a bonding apparatus used in Embodiment 1 of the present invention;

FIG. 2 is a view showing a state in which an insertion material used in this Embodiment 1 is attached to bonding surfaces of the metal materials to be bonded to each other;

FIG. 3 is a view showing a state in which an insertion material used in Embodiment 2 of the present invention is attached to bonding surfaces of the metal materials to be bonded to each other;

FIG. 4 is a schematic view of a bonding apparatus used in Embodiment 3 of the present invention;

FIG. 5 is a schematic view of a bonding apparatus used in Embodiment 5 of the present invention; and

FIG. 6 is a schematic view of a bonding apparatus used in Embodiment 7 of the present invention.

PREFERRED EMBODIMENTS OF THE INVENTION

Preferred embodiments of the present invention will be described below with reference to the accompanying drawings but the present invention is not limited to those embodiments.

Embodiment 1

FIG. 1 is a schematic view showing a main part of a bonding apparatus used in Embodiment 1 of the metal material bonding method according to the present invention. In this bonding apparatus A, metal round rods (hereinafter merely called "round rods") are used as metal materials 1 and 2 to be bonded to each other. An insertion material 3 is interposed between bonding surfaces of the round rods 1 and 2. An inert gas, such as an Ar gas, an N₂ gas, a mixture gas of Ar and N₂, or the like, is supplied from an inert gas supply source 5 into a housing 6 which receives the round rods 1 and 2 to be bonded to each other, so that the inside of the housing 6 is set in an atmosphere of inert gas. While bonding portions of the round rods 1 and 2 are pressed by a pressing device 4 and while bonding end portions of the round rods 1 and 2 are heated by an induction heating device 7 in the atmosphere of inert gas, the round rods 1 and 2 are diffusion-bonded to each other. If bonding is performed in an oxidative atmosphere, oxygen grasped between the end surfaces of the metal materials reacts with the insertion material to form oxide. If the oxide remains in the bonding surfaces, mechanical characteristic of the bonding is lowered. Accordingly, bonding is preferably performed in a non-oxidative atmosphere such as an inert gas atmosphere, or the like. Incidentally, in FIG. 1, the reference numeral 8 designates a high-frequency electric source device for the induction heating device. Further, in FIG. 1, the thickness of the insertion material 3 is exaggerated for convenience of description.

The insertion material 3 is provided as a disk formed of an Ni-group or Fe-group alloy containing boron in a range of from 2.0 mass % to 5.0 mass %. The thickness of the insertion material 3 is in a range of from 20 μm to 100 μm. Boron not only has an effect of decreasing the melting point of the Ni-group alloy but also has a high diffusion speed so as to be diffused from the dissolved insertion material 3 into a solid base material easily. An "isothermal solidification phenomenon" is generated in such a manner that the boron content of the dissolved insertion material 3 is reduced because of the diffusion of boron from the dissolved insertion material 3 into the solid base material, so that the melting point of the dissolved insertion material 3 rises. As a result of the "isothermal solidification phenomenon", the materials to be bonded to each other are fused firmly. However, if the boron content in the insertion material 3 is smaller than 2 mass %, the effect is insufficient. If the boron content is contrariwise larger than 5 mass %, the melting point of the insertion material 3 rises to simply increase the time required for diffusing boron from the dissolved insertion material 3 into the solid base material. Accordingly, in this Embodiment 1, the upper limit is set to 5 mass %. In this case, the boron content in the insertion material 3 is more preferably in a range of from 3 mass % to 4 mass %.

If the thickness of the insertion material 3 is less than 20 μm, not only it is impossible to obtain an insertion material 3 dissolved enough to make the roughness of the bonding surfaces even but also the concentration distribution of various components in the insertion material 3 becomes uneven so that it is impossible to produce any foil-like sheet in an extreme case. If the thickness of the insertion material 3 is more than 100 μm, the melted insertion material 3 is excessively extruded to the surfaces of the bonding portions by lo the "pressing force" applied to the bonding surfaces so that not only the quality of external appearance is spoiled but also the protruded portions of the insertion material 3 extruded to the surfaces of the bonding portions and solidified thereat become concentrated stress sources to deteriorate the mechanical properties of the bonding in the case where the bonding portions are used without being mechanically processed. Furthermore, the time required for diffusing boron from the melted insertion material 3 into the solid base material increases. Accordingly, the upper limit is set to 100 μm. In this Embodiment 1, the diameter of the insertion material 3 is slightly smaller than that of each of the round rods, as shown in FIG. 2. Specifically, the distance L₀ from the outer edge of the insertion material 3 to the outer edge of each of the round rods 1 and 2 is not smaller than a value ten times as large as the thickness of the insertion material 3. The ratio of the area of the insertion material 3 to the area of each of the bonding surfaces of the round rods 1 and 2 is in a range of from 50% to 99%. Here, the insertion material 3 formed of an Fe-group alloy is preferably used in the case of bonding the round rods 1 and 2 of carbon steel whereas the insertion material 3 formed of an Ni-group alloy is preferably used in the case of bonding the round rods 1 and 2 of stainless steel. Incidentally, the insertion material 3 is not necessary to be always shaped like a disk if the aforementioned condition is satisfied. For example, the insertion material 3 may be shaped like a ring. Further, in FIG. 2, the distance Lo between the outer edge of the insertion material 3 and the outer edge of the carbon steel round rod 1 (2) is exaggerated for convenience of description.

In a state in which the insertion material 3 is interposed between the bonding surfaces of the round rods 1 and 2 to be bonded to each other, the bonding surfaces are pressed by the pressing device 4. Here, the pressure applied to the round rods 1 and 2 is in a range of from 3 MPa to 9 MPa. If the pressure applied between the end surfaces of the metal materials, that is, the pressure applied between the round rods 1 and 2 is lower than 3 MPa, the dissolved insertion material 3 cannot make the roughness of the bonding surfaces even so that the mechanical properties of the bonding deteriorate. On the contrary, if the pressure applied between the end surfaces of the metal materials, that is, the pressure applied between the round rods 1 and 2 is higher than 9 MPa, the bonding portions are deformed excessively so that not only the quality of the external appearance is spoiled but also the mechanical properties of the bonding is lowered. Accordingly, the upper limit is set to 9 MPa. Further, to obtain uniform bonding, the surface roughness R_(max) of each of the end surfaces of the round rods 1 and 2 to be bonded to each other is not larger than 50 μm. If the surface roughness R_(max) of each of the bonding surfaces is larger than 50 μm, the dissolved insertion material 3 cannot make the roughness of the bonding surfaces even so that the mechanical properties of the bonding deteriorate. Accordingly, the upper limit of R_(max) is set to 50 μm.

Though not shown clearly, the induction heating device 7 has a heating portion constituted by a ring coil. The frequency of a current flowing in the ring coil should not be higher than 100 kHz. In either a high-frequency induction heating method or a high-frequency resistance heating method, only the surfaces are heated locally by the "skin effect" as the frequency becomes high. Accordingly, if the temperature outside the bonding surfaces reaches a predetermined value, the temperature of the center portion (solid material) or the inside (pipe material) of the bonding surfaces becomes excessively lower than the outside temperature, and sufficient bonding strength cannot be obtained. Accordingly, the upper limit of the frequency is set to 100 kHz. More preferably, the frequency is in a range of from 3 kHz to 30 kHz. Incidentally, the heating portion of the induction heating device 7 and the housing 6 covering the heating portion are designed so as to be able to be halved. This is preferable from the point of view of facilitating the setting and separation of the round rods 1 and 2.

As described above, in this Embodiment 1, the disk-like or ring-like insertion material 3 having a diameter slightly smaller than that of the round rods 1 and 2 is used to perform diffusion bonding. Accordingly, after bonding, the insertion material 3 is never excessively extruded to the surfaces of the bonding portions. Accordingly, no finishing is required on the bonding end portions after diffusion bonding. Accordingly, the productivity and workability in diffusion bonding are improved greatly.

Embodiment 2

In Embodiment 2 of the metal material bonding method according to the present invention, metal round pipes (hereinafter merely called "round pipes") 1 and 2 are used as the materials 1 and 2 to be bonded to each other. An insertion material 3 is used so that the round pipes 1 and 2 are bonded to each other by a bonding apparatus A shown in FIG. 1.

The insertion material 3 is provided as a ring of an Ni-group or Fe-group alloy containing boron in a range of from 2.0 mass % to 5.0 mass %. The thickness of the insertion material 3 is in a range of from 20 μm to 100 μm. In this Embodiment 2, the outer diameter of the insertion material 3 is slightly smaller than that of each of the round pipes 1 and 2, as shown in FIG. 3. Incidentally, the insertion material 3 is hatched for convenience of description. Specifically, the distance L₀ from the outer edge of the insertion material 3 to the outer edge of each of the round pipes 1 and 2 is not smaller than a value ten times as large as the thickness of the insertion material 3 whereas the distance L₁ from the inner edge of the insertion material 3 to the inner edge of each of the round pipes 1 and 2 is not larger than a value one hundred times as large as the thickness of the insertion material 3. Further, the ratio of the area of the insertion material 3 to the area of each of the bonding surfaces of the round pipes 1 and 2 is in a range of from 50% to 99%.

Incidentally, with respect to other structures and operation/effect, the Embodiment 2 is designed to be the same as the Embodiment 1. Further, in FIG. 3, the distance L₀ between the outer edge of the insertion material 3 and the outer edge of the carbon steel round pipe 1 and the distance L₁ between the inner edge of the insertion material 3 and the inner edge of the carbon steel round pipe 1 are exaggerated for convenience of description.

Embodiment 3

FIG. 4 is a schematic view showing a main part of a bonding apparatus used in Embodiment 3 of the metal material bonding method according to the present invention. In this bonding apparatus A, an insertion material 3 is interposed between bonding surfaces of round rods 1 and 2. Air in a housing 6 which receives the round rods 1 and 2 to be bonded to each other is discharged by a vacuum pump 11 so that the inside of the housing 6 is evacuated to a near vacuum in which the pressure of the inside of the housing 6 is not higher than 5×10⁻² mmHg. While the bonding portions are pressed by a pressing device 4 and while the bonding end portions are heated in the near vacuum by an induction heating device 7, the round rods 1 and 2 are diffused-bonded to each other. Incidentally, in FIG. 4, the constituent parts corresponding to those in FIG. 1 are referenced correspondingly.

With respect to the size, quality, etc., the insertion material 3 in this Embodiment 3 is made the same as the insertion material 3 in the Embodiment 1. Further, also the surface roughness of each of the bonding surfaces of the round rods 1 and 2 which is pressed by the pressing device 4 in a state in which the insertion material 3 is interposed between the bonding surfaces of the round rods 1 and 2, and the pressure and the frequency of a current for pressing the round rods 1 and 2 are made to be the same as those in the Embodiment 1, that is, the surface roughness is not larger than R_(max) =50 μm; the pressure, in a range of from 3 MPa to 9 MPa; and the frequency of a current, not higher than 100 kHz. Incidentally, the insertion material 3 may be shaped like a ring in the same manner as in the Embodiment 1.

As described above, in this Embodiment 3, the disk-like or ring-like insertion material 3 having a diameter slightly smaller than that of the round rods is used to perform diffusion bonding. Accordingly, similarly to the Embodiment 1, the insertion material 3 is never excessively extruded to the surfaces of the bonding portions after bonding. Accordingly, no finishing is required on the bonding end portions after diffusion bonding. Accordingly, the productivity and workability in diffusion bonding are improved greatly.

Embodiment 4

In Embodiment 4 of the metal material bonding method according to the present invention, round pipes 1 and 2 are bonded to each other by a bonding apparatus A shown in FIG. 4 with use of an insertion material 3.

The size and shape of the insertion material 3 in this Embodiment 4 are made to be the same as those of the insertion material 3 in the Embodiment 2.

Incidentally, with respect to other structures and operation/effect, the Embodiment 4 is made the same as the Embodiment 3.

Embodiment 5

FIG. 5 is a schematic view showing a main part of a bonding apparatus used in Embodiment 5 of the metal material bonding method according to the present invention. In this bonding apparatus A, an insertion material 3 is interposed between bonding surfaces of round rods 1 and 2. An inert gas, such as an Ar gas, an N₂ gas, a mixture gas of Ar and N₂, or the like, is supplied from an inert gas supply source 5 into a housing 6 which receives the round rods 1 and 2 to be bonded to each other, so that the inside of the housing 6 is set in an atmosphere of inert gas. While bonding portions of the round rods 1 and 2 are pressed by means of a pressing device 4, and while bonding end portions of the round rods 1 and 2 are heated in the atmosphere of inert gas by high frequency resistance heating, the round rods 1 and 2 are diffused-bonded to each other. In the drawing, the reference numeral 21 designates electrodes for high frequency resistance heating; and 22, a high-frequency electric source device. Incidentally, in FIG. 5, constituent parts corresponding to those in FIG. 1 are referenced correspondingly.

With respect to the size, quality, etc., the insertion material 3 in this Embodiment 5 is made the same as the insertion material 3 in the Embodiment 1. Further, also the surface roughness of each of the bonding surfaces of the round rods 1 and 2 which is pressed by the pressing device 4 in a state in which the insertion material 3 is interposed between the bonding surfaces of the round rods 1 and 2. The pressure and the frequency of a current for pressing the round rods 1 and 2 are made to be the same as those in the Embodiment 1. That is, the surface roughness is not larger than R_(max) =50 μm; the pressure, in a range of from 3 MPa to 9 MPa; and the frequency of a current, not higher than 100 kHz, preferably in a range of from 3 kHz to 30 kHz. Incidentally, the insertion material 3 may be shaped like a ring in the same manner as in the Embodiment 1.

As described above, in this Embodiment 5, the disk-like or ring-like insertion material 3 having a diameter slightly smaller than that of the round rods 1 and 2 is used to perform diffusion bonding. Accordingly, after bonding, the insertion material 3 is never excessively extruded to the surfaces of the bonding portions. Accordingly, no finishing is required on the bonding end portions after diffusion bonding. Accordingly, the productivity and workability in diffusion bonding are improved greatly.

Embodiment 6

In Embodiment 6 of the metal material bonding method according to the present invention, round pipes 1 and 2 are bonded to each other by a bonding apparatus A shown in FIG. 5 with use of an insertion material 3.

The size and shape of the insertion material 3 in this Embodiment 6 are made to be the same as those of the insertion material 3 in the Embodiment 2.

Incidentally, with respect to other structures and operation/effect, the Embodiment 6 is made the same as the Embodiment 5.

Embodiment 7

FIG. 6 is a schematic view showing a main part of a bonding apparatus used in Embodiment 7 of the metal material bonding method according to the present invention. In this bonding apparatus A, an insertion material 3 is interposed between bonding surfaces of round rods 1 and 2. Air in a housing 6 which receives the round rods 1 and 2 to be bonded to each other is discharged by a vacuum pump 31 so that the inside of the housing 6 is evacuated to a near vacuum in which the pressure of the inside of the housing 6 is not higher than 5×10⁻² mmHg. While the bonding portions are pressed by a pressing device 4 and while the bonding end portions are heated in the near vacuum by current conduction heating, the round rods 1 and 2 are diffusion-bonded to each other. Incidentally, in FIG. 6, the constituent parts corresponding to those in FIGS. 1 and 5 are referenced correspondingly.

With respect to the size, quality, etc., the insertion material 3 in this Embodiment 7 is made the same as the insertion material 3 in the Embodiment 1. Further, also the surface roughness of each of the bonding surfaces of the round rods 1 and 2 which is pressed by the pressing device 4 in a state in which the insertion material 3 is interposed between the bonding surfaces of the round rods 1 and 2. The pressure and the frequency of a current for pressing the round rods 1 and 2 are made to be the same as those in the Embodiment 1. That is, the surface roughness is not larger than R_(max) =50 μm; the pressure, in a range of from 3 MPa to 9 MPa; and the frequency of a current, not higher than 100 kHz, preferably in a range of from 3 kHz to 30 kHz. Incidentally, the insertion material 3 may be shaped like a ring in the same manner as in the Embodiment 1.

As described above, in this Embodiment 7, the disk-like or ring-like insertion material 3 having a diameter slightly smaller than that of the round rods 1 and 2 is used to perform diffusion bonding. Accordingly, after bonding, the insertion material 3 is never excessively extruded to the surfaces of the bonding portions. Accordingly, no finishing is required on the bonding end portions after diffusion bonding. Accordingly, the productivity and workability in diffusion bonding are improved greatly.

Embodiment 8

In Embodiment 8 of the metal material bonding method according to the present invention, round pipes 1 and 2 are bonded to each other by a bonding apparatus A shown in FIG. 6 with use of an insertion material 3.

The size and shape of the insertion material 3 in this Embodiment 8 are made to be the same as those of the insertion material 3 in the Embodiment 2.

Incidentally, with respect to other structures and operation/effect, the Embodiment 8 is made the same as the Embodiment 7.

Although the aforementioned Embodiments have shown the case where each of the metal materials to be bonded is shaped like a round rod or a round pipe, the shape of each of the metal materials to be bonded is not limited to such a round shape. But any shape other than such a round rod (pipe) may be used. For example, the metal materials may be shaped like rectangular rods or rectangular pipes. Although the Embodiments have been described above about the case of bonding in a horizontal posture by way of example, the posture is not limited to such a case but any posture, for example, an oblique posture may be used. Further, although the Embodiments have shown a structure in which a pressing force is given to bonding surfaces by pressing respective rear ends of two materials to be bonded to each other, the present invention is not limited to such a structure, but a pressing force may be given while at least one of the two materials to be bonded is being grasped.

EXAMPLES

The present invention will be described below more specifically while comparing various examples of this invention with comparative examples.

After stainless steel round rods and round pipes (SUS304) and carbon steel rectangular pipes (SS400) were bonded under various conditions respectively, a tensile test and a fatigue test were conducted. The conditions and results of the tests at that time are as follows. Here, the melting point of SUS304 is 1480° C. and the melting point of SS400 is 1495° C.

Examples 1 to 4 and Comparative Examples 1 to 3

Stainless steel round rods (SUS304: 50 mm outer diameter) shown in Table 1 were bonded to each other with use of an insertion material shown in Table 1 under a condition shown in Table 2. Then, the bonded stainless steel round rods were subjected to a tensile test and a fatigue test. Results of the test are as shown in Table 3.

It is apparent from the results shown in Table 3 that it is desirable to select the ratio of the area of the insertion material to the area of each bonding surface to be in a range of from 50% to 99%, preferably in a range of from 51% to 98%.

Incidentally, in Table 2, the "distance" and "ratio" about the "outer edge" mean the shortest distance from the outer edge of the insertion material to the outer edge of each bonding surface, and the ratio of the distance to the thickness of the insertion material, respectively. The "distance" and "ratio" about the "inner edge" mean the largest distance from the inner edge of the insertion material to the inner edge of each bonding surface, and the ratio of the distance to the thickness of the insertion material, respectively.

Further, the fatigue test was carried out by tensile compression under the condition in which the number (Nf) of repetitions was 2×10⁶ and the rate of repetition was 3 Hz.

                                      TABLE 1                                      __________________________________________________________________________                           Comp.                                                                              Comp.                                                Classification        Ex. 1                                                                              Ex. 2                                                                              Example 1                                                                           Example 2                                   __________________________________________________________________________     Material                                                                             Quality         SUS304                                                                             SUS304                                                                             SUS304                                                                              SUS304                                      to be bonded                                                                         Shape           Round                                                                              Round                                                                              Round                                                                               Round                                                             Rod Rod Rod  Rod                                               Outer Diameter (mm)                                                                            φ50                                                                            φ50                                                                            φ50                                                                             φ50                                           Inner Diameter (mm)                                                                            --  --  --   --                                          Insertion                                                                            Shape Outer Diameter (mm)                                                                      φ50                                                                            φ50                                                                            φ49.4                                                                           φ44.0                                   Material    Inner Diameter (mm)                                                                      --  --  --   --                                                      Thickness (μm)                                                                        30  30  30   30                                                Composition                                                                          Ni        Bal.                                                                               Bal.                                                                               Bal. Bal.                                              (mass %)                                                                             Si        --  --  --   --                                                      Cr        15.0                                                                               15.0                                                                               15.0 15.0                                                    Fe        --  --  --   --                                                      B         4.0 4.0 4.0  4.0                                                     C         --  --  --   --                                          __________________________________________________________________________                                        Comp.                                       Classification          Example 3                                                                           Example 4                                                                            Ex. 3                                       __________________________________________________________________________     Material                                                                              Quality          SUS304                                                                              SUS304                                                                               SUS304                                      to be bonded                                                                          Shape            Round                                                                               Round Round                                                               Rod  Rod   Rod                                                Outer Diameter (mm)                                                                             φ50                                                                             φ50                                                                              φ50                                            Inner Diameter (mm)                                                                             --   --    --                                          Insertion                                                                             Shape  Outer Diameter (mm)                                                                      φ38.0                                                                           φ40                                                                              φ35                                     Material      Inner Diameter (mm)                                                                      --   φ18                                                                              --                                                        Thickness (μm)                                                                        30   30    30                                                 Composition                                                                           Ni        Bal. Bal.  Bal.                                               (mass %)                                                                              Si        --   --    --                                                        Cr        15.0 15.0  15.0                                                      Fe        --   --    --                                                        B         4.0  4.0   4.0                                                       C         --   --    --                                          __________________________________________________________________________

                  TABLE 2                                                          ______________________________________                                                       Comp.    Comp.    Exam-  Exam-                                   Classification                                                                               Ex. 1    Ex. 2    ple 1  ple 2                                   ______________________________________                                         Position with respect                                                          to Bonding Surface                                                             Outer Edge                                                                     Distance (mm) 0.0      0.0      0.3    3.0                                     Ratio (%)     0        0        10     100                                     Inner Edge                                                                     Distance (mm) --       --       --     --                                      Ratio (%)     --       --       --     --                                      Area Ratio (%)                                                                               100      100      98     77                                      Bonding Condition                                                              Bonding Surface                                                                              30       30       30     30                                      Roughness                                                                      (Rmax, μm)                                                                  Heating                                                                        Method        Induc-   Induc-   Induc- Induc-                                                tion     tion     tion   tion                                                  Heat     Heat     Heat   Heat                                    Frequency     3        3        3      3                                       (kHz)                                                                          Bonding       1250     1250     1250   1250                                    Temperature (° C.)                                                      Holding Time (s)                                                                             60       60       60     60                                      Pressure (MPa)                                                                               4        4        4      4                                       Bonding       Ar       Ar       Ar     Ar                                      Atmosphere                                                                     ______________________________________                                                        Exam-     Exam-     Comp.                                       Classification ple 3     ple 4     Ex. 3                                       ______________________________________                                         Position with respect                                                          to Bonding Surface                                                             Outer Edge                                                                     Distance (mm)  6.0       5.0       7.5                                         Ratio (%)      200       167       250                                         Inner Edge                                                                     Distance (mm)  --        --        --                                          Ratio (%)      --        --        --                                          Area Ratio (%) 58        51        49                                          Bonding Condition                                                              Bonding Surface                                                                               30        30        30                                          Roughness                                                                      (Rmax, μm)                                                                  Heating                                                                        Method         Induc-    Induc-    Induc-                                                     tion      tion      tion                                                       Heat      Heat      Heat                                        Frequency      3         3         3                                           (kHz)                                                                          Bonding        1250      1250      1250                                        Temperature (° C.)                                                      Holding Time (s)                                                                              60        60        60                                          Pressure (MPa) 4         4         4                                           Bonding        Ar        Ar        Ar                                          Atmosphere                                                                     ______________________________________                                    

                  TABLE 3                                                          ______________________________________                                                     Comp.    Comp.                                                     Classification                                                                             Ex. 1    Ex. 2     Example 1                                                                             Example 2                                ______________________________________                                         Tensile                                                                              Tensile   640      644     642    641                                    Test  Strength                                                                       (MPa)                                                                          Rupture   Base     Base    Base   Base                                         Position  Material Material                                                                               Material                                                                              Material                               Fatigue                                                                              Fatigue   160      230     230    230                                    Test  Limit (MPa)                                                                    Rupture   Bonding  No Rupture                                                                             No     No                                           Position  Interface        Rupture                                                                               Rupture                                Evaluation  C        B         A      A                                        Remarks              Bonding                                                                        portion was                                                                    treated to be                                                                  smoothened                                                ______________________________________                                                                             Comp.                                      Classification   Example 3                                                                               Example 4 Ex. 3                                      ______________________________________                                         Tensile Test                                                                           Tensile Strength                                                                            641      642     587                                              (MPa)                                                                          Rupture Position                                                                            Base     Base    Base                                                          Material Material                                                                               Material                                 Fatigue Test                                                                           Fatigue Limit (MPa)                                                                         230      230     150                                              Rupture Position                                                                            No       No Rupture                                                                             Bonding                                                       Rupture          Interface                                Evaluation       A        A         C                                          Remarks                   Ring-like                                                                      insertion                                                                      material was                                                                   used.                                                ______________________________________                                    

Example 5 and Comparative Example 4

Stainless steel round rods (SUS304: 125 mm outer diameter) shown in Table 4 were bonded to each other with use of an insertion material shown in Table 4 under a condition shown in Table 5. Then, the bonded stainless steel round rods were subjected to a tensile test and a fatigue test. Results of the test are as shown in Table 6.

It is apparent from results shown in Table 6 that it is necessary to make the distance about the outer edge be at least ten times as large as the thickness of the insertion material.

Incidentally, in Table 5, the "distance" and "ratio" about the "outer edge" mean the shortest distance from the outer edge of the insertion material to the outer edge of each bonding surface, and the ratio of the distance to the thickness of the insertion material, respectively. The "distance" and "ratio" about the "inner edge" mean the largest distance from the inner edge of the insertion material to the inner edge of each bonding surface, and the ratio of the distance to the thickness of the insertion material, respectively.

Further, the fatigue test was carried out by tensile compression under the condition in which the number (Nf) of repetitions was 2×10⁶ and the rate of repetition was 3 Hz.

                  TABLE 4                                                          ______________________________________                                                                          Comp.                                         Classification          Example 5                                                                               Ex. 4                                         ______________________________________                                         Material                                                                               Quality             SUS304   SUS304                                    to be bonded                                                                           Shape               Round    Round                                                                 Rod      Rod                                               Outer Diameter (mm) φ125 φ125                                          Inner Diameter (mm) --       --                                        Insertion                                                                              Shape     Outer Diameter (mm)                                                                          φ124.4                                                                            φ125                                Material          Inner Diameter (mm)                                                                          --     φ12.5                                                 Thickness (μm)                                                                            30     30                                              Composition                                                                              Ni            Bal.   Bal.                                            (mass %)  Si            --     --                                                        Cr            15.0   15.0                                                      Fe            --     --                                                        B             4.0    4.0                                                       C             --     --                                      ______________________________________                                    

                  TABLE 5                                                          ______________________________________                                                                         Comp.                                          Classification         Example 5                                                                               Ex. 4                                          ______________________________________                                         Position with                                                                            Outer    Distance (mm)                                                                              0.3    0.0                                      respect to                                                                               Edge     Ratio (%)   10     0                                        Bonding Surface                                                                          Inner    Distance (mm)                                                                              --     --                                                 Edge     Ratio (%)   --     --                                                 Area Ratio (%)   99       99                                         Bonding   Bonding Surface Roughness                                                                       30       30                                         Condition (Rmax, μm)                                                                Heating                                                                               Method      Induction                                                                               Induction                                                             Heat     Heat                                                      Frequency (kHz)                                                                            3        3                                                  Bonding Temperature (° C.)                                                             1250     1250                                                   Holding Time (s)                                                                              180      180                                                    Pressure (MPa) 4        4                                                      Bonding Atmosphere                                                                            Ar       Ar                                             ______________________________________                                    

                  TABLE 6                                                          ______________________________________                                         Classification   Example 5 Comp. Ex. 4                                         ______________________________________                                         Tensile Test                                                                           Tensile Strength                                                                            643       642                                                     (MPa)                                                                          Rupture Position                                                                            Base      Base Material                                                        Material                                                  Fatigue Test                                                                           Fatigue Limit (MPa)                                                                         230       200                                                     Rupture Position                                                                            No Rupture                                                                               Bonding Interface                               Evaluation       A         B                                                   Remarks                    Rupture in bonding                                                             interface with a large                                                         fatigue limit.                                      ______________________________________                                    

Example 6 and 7 and Comparative Examples 5 and 6

Stainless steel round rods (SUS304: 125 mm outer diameter) shown in Table 7 were bonded to each other with use of an insertion material shown in the same Table 7 under a condition shown in Table 8. Then, the bonded stainless steel round rods were subjected to a tensile test and a fatigue test. Results of the test are as shown in Table 9.

It is apparent from results shown in Table 9 that it is desirable to select the boron content in the insertion material to be in a range of from 2 mass % to 5 mass %.

Incidentally, in Table 8, the "distance" and "ratio" about the "outer edge" mean the shortest distance from the outer edge of the insertion material to the outer edge of each bonding surface, and the ratio of the distance to the thickness of the insertion material, respectively. The "distance" and "ratio" about the "inner edge" mean the largest distance from the inner edge of the insertion material to the inner edge of each bonding surface, and the ratio of the distance to the thickness of the insertion material, respectively.

Further, the fatigue test was carried out by tensile compression under the condition in which the number (Nf) of repetitions was 2×10⁶ and the rate of repetition was 3 Hz.

                  TABLE 7                                                          ______________________________________                                                       Comp.    Exam-    Exam-  Comp.                                   Classification                                                                               Ex. 5    ple 6    ple 7  Ex. 6                                   ______________________________________                                         Material to be bonded                                                          Quality       SUS304   SUS304   SUS304 SUS304                                  Shape         Round    Round    Round  Round                                                 Rod      Rod      Rod    Rod                                     Outer Diameter                                                                               φ125 φ125 φ125                                                                              φ125                                (mm)                                                                           Inner Diameter                                                                               --       --       --     --                                      (mm)                                                                           Insertion Material                                                             Shape                                                                          Outer Diameter                                                                               φ122 φ122 φ122                                                                              φ122                                (mm)                                                                           Inner Diameter                                                                               --       --       --     --                                      (mm)                                                                           Thickness (μm)                                                                            30       30       30     30                                      Composition                                                                    (mass %)                                                                       Ni            Bal.     Bal.     Bal.   Bal.                                    Si            --       --       --     --                                      Cr            15.0     15.0     15.0   15.0                                    Fe            --       --       --     --                                      B             1.0      2.0      5.0    6.0                                     C             --       --       --     --                                      ______________________________________                                    

                  TABLE 8                                                          ______________________________________                                                       Comp.    Exam-    Exam-  Comp.                                   Classification                                                                               Ex. 5    ple 6    ple 7  Ex. 6                                   ______________________________________                                         Position with respect                                                          to Bonding Surface                                                             Outer Edge                                                                     Distance (mm) 1.5      1.5      1.5    1.5                                     Ratio (%)     50       50       50     50                                      Inner Edge                                                                     Distance (mm) --       --       --     --                                      Ratio (%)     --       --       --     --                                      Area Ratio (%)                                                                               95       95       95     95                                      Bonding Condition                                                              Bonding Surface                                                                              30       30       30     30                                      Roughness                                                                      (Rmax, μm)                                                                  Heating                                                                        Method        Induc-   Induc-   Induc- Induc-                                                tion     tion     tion   tion                                                  Heat     Heat     Heat   Heat                                    Frequency     3        3        3      3                                       (kHz)                                                                          Bonding       1250     1250     1250   1250                                    Temperature (° C.)                                                      Holding Time (s)                                                                             180      180      180    180                                     Pressure (MPa)                                                                               4        4        4      4                                       Bonding       Ar       Ar       Ar     Ar                                      Atmosphere                                                                     ______________________________________                                    

                  TABLE 9                                                          ______________________________________                                                      Comp.    Exam-    Exam-  Comp.                                    Classification                                                                              Ex. 5    ple 6    ple 7  Ex. 6                                    ______________________________________                                         Tensile Test                                                                   Tensile      549      641      644    588                                      Strength (MPa)                                                                 Rupture      Bonding  Base     Base   Bonding                                  Position     Interface                                                                               Material Material                                                                              Interface                                Fatigue Test                                                                   Fatigue Limit                                                                               130      230      230    200                                      (MPa)                                                                          Rupture      Bonding  No       No     Bonding                                  Position     Interface                                                                               Rupture  Rupture                                                                               Interface                                Evaluation   C        A        A      C                                        Remarks                                                                        ______________________________________                                    

Example 8 and 9 and Comparative Examples 7 and 8

Stainless steel round rods (SUS304: 125 mm outer diameter) shown in Table 10 were bonded to each other with use of an insertion material shown in the same Table 10 under a condition shown in Table 11. Then, the bonded stainless steel round rods were subjected to a tensile test and a fatigue test. Results of the test are as shown in Table 12.

It is apparent from results shown in Table 12 that it is desirable to select the thickness of the insertion material to be in a range of from 20 μm to 100 μm.

Incidentally, in Table 11, the "distance" and "ratio" about the "outer edge" mean the shortest distance from the outer edge of the insertion material to the outer edge of each bonding surface, and the ratio of the distance to the thickness of the insertion material, respectively. The "distance" and "ratio" about the "inner edge" mean the largest distance from the inner edge of the insertion material to the inner edge of each bonding surface, and the ratio of the distance to the thickness of the insertion material, respectively.

Further, the fatigue test was carried out by tensile compression under the condition in which the number (Nf) of repetitions was 2×10⁶ and the rate of repetition was 3 Hz.

                  TABLE 10                                                         ______________________________________                                                       Comp.    Exam-    Exam-  Comp.                                   Classification                                                                               Ex. 7    ple 8    ple 9  Ex. 8                                   ______________________________________                                         Material to be                                                                 bonded                                                                         Quality       SUS304   SUS304   SUS304 SUS304                                  Shape         Round    Round    Round  Round                                                 Rod      Rod      Rod    Rod                                     Outer Diameter                                                                               φ125 φ125 φ125                                                                              φ125                                (mm)                                                                           Inner Diameter                                                                               --       --       --     --                                      (mm)                                                                           Insertion Material                                                             Shape                                                                          Outer Diameter                                                                               φ122 φ122 φ122                                                                              φ122                                (mm)                                                                           Inner Diameter                                                                               --       --       --     --                                      (mm)                                                                           Thickness (μm)                                                                            10       20       100    120                                     Composition                                                                    (mass %)                                                                       Ni            Bal.     Bal.     Bal.   Bal.                                    Si            --       --       --     --                                      Cr            15.0     15.0     15.0   15.0                                    Fe            --       --       --     --                                      B             4.0      4.0      4.0    4.0                                     C             --       --       --     --                                      ______________________________________                                    

                  TABLE 11                                                         ______________________________________                                                       Comp.    Exam-    Exam-  Comp.                                   Classification                                                                               Ex. 7    ple 8    ple 9  Ex. 8                                   ______________________________________                                         Position with respect                                                          to Bonding Surface                                                             Outer Edge                                                                     Distance (mm) 1.5      1.5      1.5    1.5                                     Ratio (%)     50       50       50     50                                      Inner Edge                                                                     Distance (mm) --       --       --     --                                      Ratio (%)     --       --       --     --                                      Area Ratio (%)                                                                               95       95       95     95                                      Bonding Condition                                                              Bonding Surface                                                                              30       30       30     30                                      Roughness                                                                      (Rmax, μm)                                                                  Heating                                                                        Method        Induc-   Induc-   Induc- Induc-                                                tion     tion     tion   tion                                                  Heat     Heat     Heat   Heat                                    Frequency     3        3        3      3                                       (kHz)                                                                          Bonding       1250     1250     1250   1250                                    Temperature (° C.)                                                      Holding Time (s)                                                                             180      180      180    180                                     Pressure (MPa)                                                                               4        4        4      4                                       Bonding       Ar       Ar       Ar     Ar                                      Atmosphere                                                                     ______________________________________                                    

                  TABLE 12                                                         ______________________________________                                                     Comp.                    Comp.                                     Classification                                                                             Ex. 7    Example 8                                                                               Example 9                                                                             Ex. 8                                     ______________________________________                                         Tensile                                                                              Tensile   555      642    641    608                                     Test  Strength                                                                       (MPa)                                                                          Rupture   Bonding  Base   Base   Bonding                                       Position  Interface                                                                               Material                                                                              Material                                                                              Interface                               Fatigue                                                                              Fatigue   140      230    230    210                                     Test  Limit (MPa)                                                                    Rupture   Bonding  No     No     Bonding                                       Position  Interface                                                                               Rupture                                                                               Rupture                                                                               Interface                               Evaluation  C        A        A      C                                         Remarks                                                                        ______________________________________                                    

Example 10 and 11 and Comparative Examples 9 and 10

Stainless steel pipes (SUS304: 165 mm outer diameter×145 inner diameter) shown in Table 13 were bonded with use of an insert material shown in the same Table 13 under the condition shown in Table 14. Then, a tensile test and a fatigue test were conducted on the bonded stainless steel pipes. The results are shown in Table 15.

It is apparent from the results shown in Table 15 that it is desirable to select the ratio of the area of the insertion material to the area of each bonding surface to be in a range of from 50% to 99%, preferably in a range of from 51% to 98%.

Incidentally, in Table 14, the "distance" and "ratio" about the "outer edge" mean the shortest distance from the outer edge of the insertion material to the outer edge of each bonding surface, and the ratio of the distance to the thickness of the insertion material, respectively. The "distance" and "ratio" about the "inner edge" mean the largest distance from the inner edge of the insertion material to the inner edge of each bonding surface, and the ratio of the distance to the thickness of the insertion material, respectively.

Further, the fatigue test was carried out by tensile compression under the condition in which the number (Nf) of repetitions was 2×10⁶ and the rate of repetition was 3 Hz.

                  TABLE 13                                                         ______________________________________                                                       Comp.    Exam-    Exam-  Comp.                                   Classification                                                                               Ex. 9    ple 10   ple 11 Ex. 10                                  ______________________________________                                         Material to be                                                                 bonded                                                                         Quality       SUS304   SUS304   SUS304 SUS304                                  Shape         Pipe     Pipe     Pipe   Pipe                                    Outer Diameter                                                                               φ165 φ165 φ165                                                                              φ165                                (mm)                                                                           Inner Diameter                                                                               φ145 φ145 φ145                                                                              φ145                                (mm)                                                                           Insertion Material                                                             Shape                                                                          Outer Diameter                                                                               φ165 φ164.2                                                                              φ164.2                                                                            φ155                                (mm)                                                                           Inner Diameter                                                                               φ145 φ145 φ154.4                                                                            φ145                                (mm)                                                                           Thickness     40       40       40     40                                      (μm)                                                                        Composition                                                                    (mass %)                                                                       Ni            Bal.     Bal.     Bal.   Bal.                                    Si            4.0      4.0      4.0    4.0                                     Cr            8.0      8.0      8.0    8.0                                     Fe            3.0      3.0      3.0    3.0                                     B             4.0      4.0      4.0    4.0                                     C             --       --       --     --                                      ______________________________________                                    

                  TABLE 14                                                         ______________________________________                                                       Comp.    Exam-    Exam-  Comp.                                   Classification                                                                               Ex. 9    ple 10   ple 11 Ex. 10                                  ______________________________________                                         Position with respect                                                          to Bonding Surface                                                             Outer Edge                                                                     Distance (mm) 0.0      0.4      0.4    5.0                                     Ratio (%)     0        10       10     125                                     Inner Edge                                                                     Distance (mm) 0.0      0.0      0.7    0.0                                     Ratio (%)     0        0        18     0                                       Area Ratio (%)                                                                               100      96       50     48                                      Bonding Condition                                                              Bonding Surface                                                                              30       30       30     30                                      Roughness                                                                      (Rmax, μm)                                                                  Heating                                                                        Method        Induc-   Induc-   Induc- Induc-                                                tion     tion     tion   tion                                                  Heat     Heat     Heat   Heat                                    Frequency     3        3        3      3                                       (kHz)                                                                          Bonding       1250     1250     1250   1250                                    Temperature (° C.)                                                      Holding Time (s)                                                                             120      120      120    120                                     Pressure (MPa)                                                                               4        4        4      4                                       Bonding       Ar       Ar       Ar     Ar                                      Atmosphere                                                                     ______________________________________                                    

                  TABLE 15                                                         ______________________________________                                                     Comp.    Example  Example                                                                               Comp.                                     Classification                                                                             Ex. 9    10       11     Ex. 10                                    ______________________________________                                         Tensile                                                                              Tensile   648      643    640    639                                     Test  Strength                                                                       (MPa)                                                                          Rupture   Base     Base   Base   Base                                          Position  Material Material                                                                              Material                                                                              Material                                Fatigue                                                                              Fatigue   210      230    230    220                                     Test  Limit (MPa)                                                                    Rupture   Bonding  No     No     Bonding                                       Position  Interface                                                                               Rupture                                                                               Rupture                                                                               Interface                               Evaluation  B        A        A      B                                         Remarks                              Rupture in                                                                     bonding                                                                        interface                                                                      with a large                                                                   fatigue limit.                            ______________________________________                                    

Example 12 and 13 and Comparative Examples 11 and 12

Stainless steel pipes (SUS304: 165 mm outer diameter×145 inner diameter) shown in Table 16 were bonded with use of an insert material shown in the same Table 16 under the condition shown in Table 17. Then, a tensile test and a fatigue test were conducted on the bonded stainless steel pipes. The results are shown in Table 18.

It is apparent from results shown in Table 18 that it is desirable to select the thickness of the insertion material to be in a range of from 20 μm to 100 μm.

Incidentally, in Table 17, the "distance" and "ratio" about the "outer edge" mean the shortest distance from the outer edge of the insertion material to the outer edge of each bonding surface, and the ratio of the distance to the thickness of the insertion material, respectively. The "distance" and "ratio" about the "inner edge" mean the largest distance from the inner edge of the insertion material to the inner edge of each bonding surface, and the ratio of the distance to the thickness of the insertion material, respectively.

Further, the fatigue test was carried out by tensile compression under the condition in which the number (Nf) of repetitions was 2×10⁶ and the rate of repetition was 3 Hz.

                  TABLE 16                                                         ______________________________________                                                       Exam-    Exam-    Comp.  Comp.                                   Classification                                                                               ple 12   ple 13   Ex. 11 Ex. 12                                  ______________________________________                                         Material to be                                                                 bonded                                                                         Quality       SUS304   SUS304   SUS304 SUS304                                  Shape         Pipe     Pipe     Pipe   Pipe                                    Outer Diamter φ165 φ165 φ165                                                                              φ165                                (mm)                                                                           Inner Diameter                                                                               φ145 φ145 φ145                                                                              φ145                                (mm)                                                                           Insertion Material                                                             Shape                                                                          Outer Diameter                                                                               φ160 φ160 φ160                                                                              φ164.2                              (mm)                                                                           Inner Diameter                                                                               φ146 φ146 φ146                                                                              φ154.4                              (mm)                                                                           Thickness (μm)                                                              Composition                                                                    (mass %)                                                                       Ni            Bal.     Bal.     Bal.   Bal.                                    Si            4.0      4.0      4.0    4.0                                     Cr            8.0      8.0      8.0    8.0                                     Fe            3.0      3.0      3.0    3.0                                     B             2.0      5.0      4.0    4.0                                     C             --       --       --     --                                      ______________________________________                                    

                  TABLE 17                                                         ______________________________________                                                       Exam-    Exam-    Comp.  Comp.                                   Classification                                                                               ple 12   ple 13   Ex. 11 Ex. 12                                  ______________________________________                                         Position with respect                                                          to Bonding Surface                                                             Outer Edge                                                                     Distance (mm) 2.5      2.5      2.5    2.5                                     Ratio (%)     63       63       63     63                                      Inner Edge                                                                     Distance (mm) 0.5      0.5      0.5    0.5                                     Ratio (%)     13       13       13     13                                      Area Ratio (%)                                                                               69       69       69     69                                      Bonding Condition                                                              Bonding Surface                                                                              30       30       30     30                                      Roughness                                                                      (Rmax, μm)                                                                  Heating                                                                        Method        Induc-   Induc-   Induc- Induc-                                                tion     tion     tion   tion                                                  Heat     Heat     Heat   Heat                                    Frequency     3        3        3      3                                       (kHz)                                                                          Bonding       1250     1250     1250   1250                                    Temperature (° C.)                                                      Holding Time (s)                                                                             120      120      120    120                                     Pressure (MPa)                                                                               4        4        4      4                                       Bonding       Ar       Ar       Ar     Ar                                      Atmosphere                                                                     ______________________________________                                    

                  TABLE 18                                                         ______________________________________                                                     Example  Example  Comp.  Comp.                                     Classification                                                                             12       13       Ex. 11 Ex. 12                                    ______________________________________                                         Tensile                                                                              Tensile   634      646    567    597                                     Test  Strength                                                                       (MPa)                                                                          Rupture   Base     Base   Bonding                                                                               Bonding                                       Position  Material Material                                                                              Interface                                                                             Interface                               Fatigue                                                                              Fatigue   230      230    160    180                                     Test  Limit (MPa)                                                                    Rupture   No       No     Bonding                                                                               Bonding                                       Position  Rupture  Rupture                                                                               Interface                                                                             Interface                               Evaluation  A        A        C      C                                         Remarks                                                                        ______________________________________                                    

Example 14 and Comparative Example 13

Carbon steel rectangular pipes (SS400: 100 mm square×80 mm square) shown in Table 19 were bonded to each other with use of an insertion material shown in the same Table 19 under a condition shown in Table 20. Then, the bonded carbon steel rectangular pipes were subjected to a tensile test and a fatigue test. Results of the test are as shown in Table 21.

It is apparent from results shown in Table 21 that it is necessary to select the surface roughness R_(max) of each bonding surface to be not larger than 50 μm.

Incidentally, in Table 20, the "distance" and "ratio" about the "outer edge" mean the shortest distance from the outer edge of the insertion material to the outer edge of each bonding surface, and the ratio of the distance to the thickness of the insertion material, respectively. The "distance" and "ratio" about the "inner edge" mean the largest distance from the inner edge of the insertion material to the inner edge of each bonding surface, and the ratio of the distance to the thickness of the insertion material, respectively.

Further, the fatigue test was carried out by tensile compression under the condition in which the number (Nf) of repetitions was 2×10⁶ and the rate of repetition was 3 Hz.

                  TABLE 19                                                         ______________________________________                                                              Comp.                                                     Classification       Ex. 13    Example 14                                      ______________________________________                                         Material                                                                               Quality          SS400     SS400                                       to be bonded                                                                           Shape            Rectangular                                                                              Rectangular                                                          Pipe      Pipe                                                Outer Size (mm)  □100                                                                          □100                                     Inner Size (mm)  □80                                                                           □80                              Insertion                                                                              Shape     Outer Size □98.8                                                                       □98.8                          Material          (mm)                                                                           Inner Size □80                                                                         □80                                              (mm)                                                                           Thickness (μm)                                                                         30      30                                                Composition                                                                              Ni         --      --                                                (mass %)  Si         3.0     3.0                                                         Cr         --      --                                                          Fe         Bal.    Bal.                                                        B          3.0     3.0                                                         C          1.5     1.5                                       ______________________________________                                    

                  TABLE 20                                                         ______________________________________                                                               Comp.                                                    Classification        Ex. 13   Example 14                                      ______________________________________                                         Position with                                                                            Outer   Distance (mm)                                                                              0.6    0.6                                       respect to                                                                               Edge    Ratio (%)   20     20                                        Bonding Surface                                                                          Inner   Distance (mm)                                                                              0.0    0.0                                                 Edge    Ratio (%)   0      0                                                   Area Ratio (%)  66       66                                          Bonding   Bonding Surface Rough-                                                                         60       50                                          Condition ness (Rmax, μm)                                                           Heating                                                                              Method      Induction                                                                               Induction                                                             Heat     Heat                                                      Frequency (kHz)                                                                            3        3                                                   Bonding Temperature                                                                          1250     1250                                                    (° C.)                                                                  Holding Time (s)                                                                             60       60                                                      Pressure (MPa)                                                                               4        4                                                       Bonding Atmosphere                                                                           Ar       Ar                                              ______________________________________                                    

                  TABLE 21                                                         ______________________________________                                         Classification    Comp. Ex. 13                                                                               Example 14                                       ______________________________________                                         Tensile Test                                                                           Tensile Strength                                                                             430         450                                                  (MPa)                                                                          Rupture Position                                                                             Bonding Interface                                                                          Base Material                                Fatigue Test                                                                           Fatigue Limit (MPa)                                                                          210         270                                                  Rupture Position                                                                             Bonding Interface                                                                          No Rupture                                   Evaluation        C           A                                                Remarks                                                                        ______________________________________                                    

Example 15 and 16 and Comparative Examples 14 and 15

Carbon steel rectangular pipes (SS400: 100 mm square×80 mm square) shown in Table 22 were bonded to each other with use of an insertion material shown in the same Table 22 under a condition shown in Table 23. Then, the bonded carbon steel rectangular pipes were subjected to a tensile test and a fatigue test. Results of the test are as shown in Table 24.

It is apparent from results shown in Table 24 that it is desirable to select the pressure to be applied to be in a range of from 3 MPa to 9 MPa.

Incidentally, in Table 23, the "distance" and "ratio" about the "outer edge" mean the shortest distance from the outer edge of the insertion material to the outer edge of each bonding surface, and the ratio of the distance to the thickness of the insertion material, respectively. The "distance" and "ratio" about the "inner edge" mean the largest distance from the inner edge of the insertion material to the inner edge of each bonding surface, and the ratio of the distance to the thickness of the insertion material, respectively.

Further, the fatigue test was carried out by tensile compression under the condition in which the number (Nf) of repetitions was 2×10⁶ and the rate of repetition was 3 Hz.

                  TABLE 22                                                         ______________________________________                                                       Comp.    Exam-    Exam-  Comp.                                   Classification                                                                               Ex. 14   ple 15   ple 16 Ex. 15                                  ______________________________________                                         Material to be bonded                                                          Quality       SS400    SS400    SS400  SS400                                   Shape         Rectan-  Rectan-  Rectan-                                                                               Rectan-                                               gular    gular    gular  gular                                                 Pipe     Pipe     Pipe   Pipe                                    Outer Size    □100                                                                         □100                                                                         □100                                                                       □100                         (mm)                                                                           Inner Size    □80                                                                          □80                                                                          □80                                                                        □80                          (mm)                                                                           Insertion Material                                                             Shape                                                                          Outer Size    □98.8                                                                        □98.8                                                                        □98.8                                                                      □98.8                        (mm)                                                                           Inner Size    □80                                                                          □80                                                                          □80                                                                        □80                          (mm)                                                                           Thickness     30       30       30     30                                      (μm)                                                                        Composition                                                                    (mass %)                                                                       Ni            --       --       --     --                                      Si            3.0      3.0      3.0    3.0                                     Cr            --       --       --     --                                      Fe            Bal.     Bal.     Bal.   Bal.                                    B             3.0      3.0      3.0    3.0                                     C             1.5      1.5      1.5    1.5                                     ______________________________________                                    

                  TABLE 23                                                         ______________________________________                                                       Comp.    Exam-    Exam-  Comp.                                   Classification                                                                               Ex. 14   ple 15   ple 16 Ex. 15                                  ______________________________________                                         Position with respect                                                          to Bonding Surface                                                             Outer Edge                                                                     Distance (mm) 0.6      0.6      0.6    0.6                                     Ratio (%)     20       20       20     20                                      Inner Edge                                                                     Distance (mm) 0.0      0.0      0.0    0.0                                     Ratio (%)     0        0        0      0                                       Area Ratio (%)                                                                               66       66       66     66                                      Bonding Condition                                                              Bonding Surface                                                                              30       30       30     30                                      Roughness                                                                      (Rmax, μm)                                                                  Heating                                                                        Method        Induc-   Induc-   Induc- Induc-                                                tion     tion     tion   tion                                                  Heat     Heat     Heat   Heat                                    Frequency     3        3        3      3                                       (kHz)                                                                          Bonding       1250     1250     1250   1250                                    Temperature (° C.)                                                      Holding Time (s)                                                                             60       60       60     60                                      Pressure (MPa)                                                                               2        3        9      10                                      Bonding       N.sub.2  N.sub.2  N.sub.2                                                                               N.sub.2                                 Atmosphere                                                                     ______________________________________                                    

                  TABLE 24                                                         ______________________________________                                                    Comp.     Exam-    Exam-  Comp.                                     Classification                                                                            Ex. 14    ple 15   ple 16 Ex. 15                                    ______________________________________                                         Tensile Test                                                                   Tensile    388       447      445    420                                       Strength (MPa)                                                                 Rupture    Bonding   Base     Base   Base                                      Position   Inter-    Ma-      Ma-    Ma-                                                  face      terial   terial terial                                    Fatigue Test                                                                   Fatigue Limit                                                                             170       270      270    200                                       (MPa)                                                                          Rupture    Bonding   No       No     Bonding                                   Position   Inter-    Rup-     Rup-   Inter-                                               face      ture     ture   face                                      Evaluation x         ∘                                                                           ∘                                                                         Δ                                   Remarks                              Ex-                                                                            cessive                                                                        Defor-                                                                         mation                                                                         in                                                                             Bonding                                                                        Portion.                                  ______________________________________                                    

Examples 17 to 19 and Comparative Examples 16 to 18

Carbon steel rectangular pipes (SS400: 100 mm square×80 mm square) shown in Table 25 were bonded to each other with use of an insertion material shown in the same Table 25 under a condition shown in Table 26. Then, the bonded carbon steel rectangular pipes were subjected to a tensile test and a fatigue test. Results of the test are as shown in Table 27.

It is apparent from results shown in Table 27 that it is preferable to carry out bonding in a vacuum or in a nitrogen gas atmosphere under the condition in which the frequency of a current is in a range of from 3 kHz to 100 kHz.

Incidentally, in Table 26, the "distance" and "ratio" about the "outer edge" mean the shortest distance from the outer edge of the insertion material to the outer edge of each bonding surface, and the ratio of the distance to the thickness of the insertion material, respectively. The "distance" and "ratio" about the "inner edge" mean the largest distance from the inner edge of the insertion material to the inner edge of each bonding surface, and the ratio of the distance to the thickness of the insertion material, respectively.

Further, the fatigue test was carried out by tensile compression under the condition in which the number (Nf) of repetitions was 2×10⁶ and the rate of repetition was 3 Hz.

                  TABLE 25                                                         ______________________________________                                                  Exam-   Comp.   Exam- Comp. Exam- Comp.                               Classification                                                                          ple 17  Ex. 16  ple 18                                                                               Ex. 17                                                                               ple 19                                                                               Ex. 18                              ______________________________________                                         Material to                                                                    be bonded                                                                      Quality  SS400   SS400   SS400 SS400 SS400 SS400                               Shape    Rectan- Rectan- Rectan-                                                                              Rectan-                                                                              Rectan-                                                                              Rectan-                                      gular   gular   gular gular gular gular                                        Pipe    Pipe    Pipe  Pipe  Pipe  Pipe                                Outer Size                                                                              □100                                                                        □100                                                                        □100                                                                      □100                                                                      □100                                                                      □100                     (mm)                                                                           Inner Size                                                                              □80                                                                         □80                                                                         □80                                                                       □80                                                                       □80                                                                       □80                      (mm)                                                                           Insertion                                                                      Material                                                                       Shape                                                                          Outer Size                                                                              □98.8                                                                       □98.8                                                                       □98.8                                                                     □98.8                                                                     □98.8                                                                     □98.8                    (mm)                                                                           Inner Size                                                                              □80                                                                         □80                                                                         □80                                                                       □80                                                                       □80                                                                       □80                      (mm)                                                                           Thickness                                                                               30      30      30    30    30    30                                  (μm)                                                                        Composition                                                                    (mass %)                                                                       Ni       --      --      --    --    --    --                                  Si       3.0     3.0     3.0   3.0   3.0   3.0                                 Cr       --      --      --    --    --    --                                  Fe       Bal.    Bal.    Bal.  Bal.  Bal.  Bal.                                B        3.0     3.0     3.0   3.0   3.0   3.0                                 C        1.5     1.5     1.5   1.5   1.5   1.5                                 ______________________________________                                    

                  TABLE 26                                                         ______________________________________                                                  Exam-   Comp.   Exam- Comp. Exam- Comp.                               Classification                                                                          ple 17  Ex. 16  ple 18                                                                               Ex. 17                                                                               ple 19                                                                               Ex. 18                              ______________________________________                                         Position with                                                                  respect to                                                                     Bonding                                                                        Surface                                                                        Outer Edge                                                                     Distance 0.6     0.6     0.6   0.6   0.6   0.6                                 (mm)                                                                           Ratio (%)                                                                               20      20      20    20    20    20                                  Inner Edge                                                                     Distance 0.0     0.0     0.0   0.0   0.0   0.0                                 (mm)                                                                           Ratio (%)                                                                               0       0       0     0     0     0                                   Area Ratio                                                                              66      66      66    66    66    66                                  (%)                                                                            Bonding                                                                        Condition                                                                      Bonding  30      30      30    30    30    30                                  Surface                                                                        Roughness                                                                      (Rmax, μm)                                                                  Heating                                                                        Method   Induc-  Induc-  Induc-                                                                               Induc-                                                                               Induc-                                                                               Induc-                                       tion    tion    tion  tion  tion  tion                                         Heat    Heat    Heat  Heat  Heat  Heat                                Frequency                                                                               3       3       100   200   100   200                                 (kHz)                                                                          Bonding  1250    1250    1250  1250  1250  1250                                Temperature                                                                    (° C.)                                                                  Holding  60      60      60    60    60    60                                  Time (s)                                                                       Pressure 4       4       4     4     4     4                                   (MPa)                                                                          Bonding  Vacuum  Air     N.sub.2                                                                              N.sub.2                                                                              Vacuum                                                                               Vacuum                              Atmosphere                                                                     ______________________________________                                    

                  TABLE 27                                                         ______________________________________                                         Classi-                                                                              Exam-   Comp.    Exam- Comp.  Exam- Comp.                                fication                                                                             ple 17  Ex. 16   ple 18                                                                               Ex. 17 ple 19                                                                               Ex. 18                               ______________________________________                                         Tensile                                                                        Test                                                                           Tensile                                                                              444     321      449   399    443   401                                  Strength                                                                       (MPa)                                                                          Rupture                                                                              Base    Bonding  Base  Bonding                                                                               Base  Bonding                              Position                                                                             Ma-     Inter-   Ma-   Inter- Ma-   Inter-                                     terial  face     terial                                                                               face   terial                                                                               face                                 Fatigue                                                                        Test                                                                           Fatigue                                                                              270     110      270   180    270   190                                  Limit                                                                          (MPa)                                                                          Rupture                                                                               No     Bonding  No    Bonding                                                                               No    Bonding                              Position                                                                             Rup-    Inter-   Rup-  Inter- Rup-  Inter-                                     ture    face     ture  face   ture  face                                 Evalua-                                                                              A       C        A     C      A     C                                    tion                                                                           Re-                                                                            marks                                                                          ______________________________________                                    

As described above in detail, according to the present invention, the insertion material is prevented from being partly excessively extruded to bonding end portions and solidified thereat, at the time of bonding. Accordingly, there arises an excellent effect that both the workability and productivity in metal material bonding can be improved. Further, because the insertion material is prevented from being partly excessively extruded to bonding end portions and solidified thereat, at the time of bonding, there arises an excellent effect that the fatigue strength against the metal materials bonded to each other is never lowered. 

What is claimed is:
 1. A metal material bonding method comprising the steps of:interposing an insertion material, which has a lower melting point than that of metal materials to be bonded, between bonding end surfaces of the metal materials to be bonded; and heating and holding the metal materials to be bonded to a temperature not lower than a melting point of the insertion material and not higher than a melting point of the metal materials to be bonded while applying pressure to bonding surfaces of the metal materials to be bonded; wherein the insertion material has a size which is smaller than each of the bonding surfaces of the metal materials to be bonded such that there is a distance between an outer edge of the insertion material and an outer edge of the metal materials to be bonded.
 2. A metal material bonding method according to claim 1, wherein each of the metal materials to be bonded is a solid material;the insertion material is formed of an Ni-group or Fe-group alloy containing boron in a range of from 2.0 mass % to 5.0 mass % and has a thickness in a range of from 20 μm to 100 μm; a ratio of the area of the insertion material to the area of each of the bonding surfaces of the metal materials to be bonded is in a range of from 50% to 99%; and the distance between the outer edge of the insertion material and the outer edge of each of the metal materials to be bonded is not smaller than a value ten times as large as the thickness of the insertion material.
 3. A metal material bonding method according to claim 1, wherein each of the metal materials to be bonded is a hollow material;the insertion material is formed of an Ni-group or Fe-group alloy containing boron in a range of from 2.0 mass % to 5.0 mass % and has a thickness in a range of from 20 μm to 100 μm; a ratio of the area of the insertion material to the area of each of the bonding surfaces of the metal materials to be bonded is in a range of from 50% to 99%; the distance between the outer edge of the insertion material and the outer edge of each of the metal materials to be bonded is not smaller than a value ten times as large as the thickness of the insertion material; and a distance between an inner edge of the insertion material and an inner edge of each of the metal materials to be bonded is not larger than a value one hundred times as large as the thickness of the insertion material.
 4. A metal material bonding method according to claim 1, wherein the surface roughness R_(max) of each of the bonding surfaces of the metal materials to be bonded is not larger than 50 μm;the pressure applied to the bonding surfaces of the metal materials to be bonded is in a range of from 3 MPa to 9 Mpa; and the heating and holding is performed in a non-oxidative atmosphere.
 5. A metal material bonding method according to claim 4, wherein the heating and holding is performed in an atmosphere of inert gas.
 6. A metal material bonding method according to claim 5, wherein the inert gas is selected from an Ar gas, an N₂ gas and a mixture gas of Ar and N₂.
 7. A metal material bonding method according to claim 4, wherein the heating and holding is performed in a substantial vacuum.
 8. A metal material bonding method according to claim 7, wherein the substantial vacuum has a pressure not higher than 5×10⁻² mmHg.
 9. A metal material bonding method according to claim 1, wherein the heating is performed by induction heating or high frequency resistance heating.
 10. A metal material bonding method according to claim 9, wherein the frequency of a current in the induction heating or resistance heating is in a range of from 3 kHz to 100 kHz.
 11. A metal material bonding method according to claim 2, wherein the insertion material is formed of an Ni-group or Fe-group alloy containing boron in a range of from 3.0 mass % to 4.0 mass %.
 12. A metal material bonding method according to claim 3, wherein the insertion material is formed of an Ni-group or Fe-group alloy containing boron in a range of from 3.0 mass % to 4.0 mass %.
 13. A metal material bonding method according to claim 10, wherein the frequency of the current in the induction heating or resistance heating is in a range of from 3 kHz to 30 kHz. 